Fiber Curl

Fiber curl is the inherent curvature along a specific length of optical fiber that is exhibited to some degree by all fibers. It is a result of thermal stresses that occur during the manufacturing process. Therefore, these factors must be rigorously monitored and controlled during fiber manufacture. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be misaligned during splicing, thereby impacting splice loss.

Typical mass fusion splicers use fixed v-grooves for fiber alignment, where the effect of fiber curl is most noticeable.

Fig 8. Cladding Diameter, Core/Clad Concentricity, and Fiber Curl

 

 

6. How to Choose Optical Fiber

Single-Mode Fiber Performance Characteristics

The key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter.

Optical fiber performance parameters can vary significantly among fibers from different manufacturers, in ways that can affect your system's performance. It is important to understand how to specify the fiber that best meets system requirements.

Attenuation

Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km).

Optical fiber offers superior performance over other transmission media because it combines high bandwidth with low attenuation. This allows signals to be transmitted over longer distances while using fewer regenerators (amplifiers), reducing cost, and improving reliability.

Dispersion

Dispersion is the smearing or broadening of an optical signal that results from the many discrete wavelength components traveling at different rates (see Figure 9). In digital transmission, dispersion limits the maximum data rate or information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO).

Figure 9. Signal Dispersion

 

 

Dispersion vs. Wavelength

Fiber dispersion varies with wavelength and is controlled by fiber design (see Figure 10). The wavelength at which dispersion equals zero is called the zero-dispersion wavelength. This is the wavelength at which fiber has its maximum information-carrying capacity. For standard single-mode fibers, this is in the region of 1310 nm.

Figure 10. Dispersion and Wavelength

Dispersion is expressed as the time increase in signal width (in picoseconds) per unit divided by the source spectral width (in nm) per unit times the length of fiber (in km).

Chromatic dispersion consists of two kinds of dispersion. Material dispersion refers to the pulse spreading caused by the specific composition of the glass. Waveguide dispersion is the pulse spreading that occurs as the light travels in both the core and the inner cladding glasses. The two types can be balanced to produce a wavelength of zero dispersion at 1310 nm.

Dispersion-Shifted Fiber

Optical fibers also can be manufactured to have the zero dispersion wavelength in the 1550-nm region, which coincides with fiber's lowest attenuation point. Dispersion-shifted fiber can allow for greater transmission capacity over longer distances than would be possible with standard single-mode fiber.

Transmission in the 1550 nm Window

Optical fibers also can be manufactured to have the zero dispersion wavelength in the 1550-nm region, which is also the point where silica-based fibers have inherently minimal attenuation. These fibers are referred to as nonzero dispersion-shifted fiber (NZDSF). This fiber is used primarily in applications that require both long-distance and high-capacity transmission rates.

For applications such as the interconnection of headends, delivery of programming to remote node sites, high-speed communication networks, and regional and metropolitan rings (used primarily for competitive access applications), NZDSF fiber can improve system reliability, increase capacity, and lower system costs (see Figure 11).

Figure 11. Wavelength and Dispersion